1. Technical Field
The disclosed embodiments relate to operational amplifiers, and more particularly relate to operational amplifiers suitable for rail-to-rail operation in analog/mixed signal integrated circuits.
2. Background Information
FIG. 1 (Prior Art) is a diagram of a mixed signal integrated circuit 1. Mixed signal integrated circuit 1 includes a digital logic portion 2 and an analog circuitry portion 3. The analog circuitry may, for example, include circuitry whose analog signals swing over voltage ranges that are bigger than the swings of the digital signals within the digital logic portion 2.
FIG. 2 (Prior Art) is a circuit diagram of a representative complementary logic digital inverter 4 within portion 2. Inverter 4 includes a thin gate insulator P-channel transistor 5 and a thin gate insulator N-channel transistor 6. These thin gate insulator transistors are sometimes referred to as “baseline devices” because they are the standard logic transistors that make up the majority of the transistors on integrated circuit 1. In the illustrated example, the digital logic within portion 2 operates from a digital logic supply voltage referred to here as VDD. Supply voltage VDD in this example ranges from about 1.0 volts to 1.3 volts. The thin gate insulator transistors have drain-to-source and gate-to-source breakdown voltages of approximately 2.0 volts or are otherwise specified to operate with drain-to-source voltages (Vds) and a gate-to-source voltages (Vgs) of 2.0 volts or less. Because supply voltage VDD is less than 2.0 volts, the complementary logic circuitry such as the circuit of FIG. 2 operates satisfactorily and does not suffer reliability problems due to overstressing of the logic transistors.
The circuitry of mixed signal integrated circuit 1, however, also includes analog circuitry in portion 3. The analog circuitry may, for example, include input/output (I/O) circuitry for interfacing the digital logic to other circuitry outside integrated circuit 1. Such analog circuitry typically operates from a higher supply voltage. The higher supply voltage is referred to here as analog supply voltage VDDA.
FIG. 3 (Prior Art) is an example of such an analog circuit referred to here as a “rail-to-rail operational amplifier” 7. In this example of an application of the rail-to-rail amplifier 7, a single-ended digital input signal VIN is supplied onto the non-inverting differential input lead 8. The digital signal has a voltage swing of from approximately ground potential to the digital logic supply voltage VDD (in this example, from ground potential to 1.3 volts). Dashed line 16 represents a negative feedback loop. Output signal VOUT on output lead 9 has a voltage swing from approximately ground potential to the analog supply voltage VDDA (in this example, from ground potential to 2.6 volts). Operational amplifier 7 includes a differential input stage 10 and an output stage 11. Differential input stage 10 includes a differential input circuit 12 and a class AB control circuit 13.
FIG. 4 (Prior Art) shows the output stage 11 of the operational amplifier 7 of FIG. 3 in further detail. Output stage 11 includes a P-channel transistor 14 and an N-channel transistor 15. Because the voltage of the output signal VOUT on output lead 9 can range from ground potential to 2.7 volts, using baseline thin gate insulator devices for transistors 14 and 15 that have breakdown voltage ratings of 2.0 volts would subject the devices to overstress. Transistors 14 and 15 could experience drain-to-source voltages (Vds) of 2.7 volts, when the baseline devices have rated drain-to-source breakdown voltages (Vdsbd) of approximately 2.0 volts. Transistors 14 and 15 are therefore made to be thick gate insulator transistors that have higher breakdown voltages. In one example, the thick gate insulator transistors have a Vdsbd breakdown voltage of approximately 3.0 volts. They are therefore able to withstand the stresses imposed by the higher analog supply voltage VDDA range signals on output lead 9.
The circuitry of FIGS. 1-4 works well. Unfortunately, providing transistors with two different gate insulator thicknesses increases the processing cost of making the mixed signal integrated circuit 1. Fabricating the thick gate insulator output stage transistors 14 and 15 generally requires using additional lithography masks, and requires carrying out multiple additional semiconductor fabrication processing steps. Due to this additional complexity, the cost of making mixed signal integrated circuit 1 may be increased by as much as five percent or more due to the requirement of having to provide the thick oxide output stage transistors.